1. Field of the Invention
The invention relates to an image pickup apparatus which is suitable for use in a video apparatus such as video camera, electronic still camera, or the like.
2. Related Background Art
In recent years, the development of video apparatus including video cameras, electronic still cameras, and the like has been remarkable. Particularly, in order to improve the function and operating efficiency, the functions such as automatic focal point adjusting apparatus (AF) and the like have been installed as standard functions.
In the automatic focal point adjusting apparatus, a method whereby a sharpness of the screen is detected from a video signal obtained by photoelectrically converting an object image by an image pickup device or the like and a focusing lens position is controlled so as to obtain the maximum sharpness to thereby execute a focal point adjustment is main stream.
Generally, a magnitude of a high frequency component of a video signal extracted by a band pass filter (BPF), a detection magnitude of an unfocused width (width of the edge portion of an object image) of a video signal extracted by a differentiating circuit or the like is used as a value for evaluation of the sharpness.
When an ordinary object image has been photographed, a level of a high frequency component is small and an unfocused width is blurred and widened in an unfocused state. As a focusing state approaches an in-focus state, the level of high frequency component rises and the unfocused width decreases. In a complete in-focus state, the high frequency component level becomes maximum and the unfocused width becomes minimum. Therefore, the focusing lens is controlled in a manner such that when the sharpness is low, the focusing lens is driven at the highest possible speed so as to raise the sharpness, the driving speed is reduced as the sharpness rises, and the driving of the focusing lens is stopped at a summit of the mountain of sharpness with a high precision. Such a control method is generally called a mountain climbing auto focusing method (mountain climbing AF).
By using such an automatic focal point adjusting apparatus, particularly, in a conventional video camera or the like to photograph a moving image, the operating efficiency is remarkably improved and such an AF is an inevitable function in recent years.
According to the automatic focal point adjusting apparatus of the mountain climbing AF method as mentioned above, however, for instance, in the case where a power source has been turned on with a lens cap attached or in the case where an object is photographed in a state of a low illuminance, a sharpness signal of a photographed image deteriorates as a whole and it is difficult to detect a focusing degree.
Thus, there is a problem such that a focus motor cannot be stopped at an in-focus point with a high precision and what is called a hunting occurs.
Such a hunting state gives an extremely unpleasant feeling to a photographing person and, further, causes troubles such as increases in electric power consumption and motor driving noises, deterioration in durability of the focusing driving system, and the like.